Typical encapsulants for LEDs are organic polymeric materials. Encapsulant lifetime is a significant hurdle holding back improved performance of high brightness LEDs. Conventional LEDs are encapsulated in epoxy resins and, when in use, tend to yellow over time reducing the LED brightness and changing the color rendering index of the light emitted from the light emitting device. This is particularly important for white LEDs. The yellowing of the epoxy is believed to result from decomposition induced by the high operating temperatures of the LED and/or absorption of UV-blue light emitted by the LED.
A second problem that can occur when using conventional epoxy resins is stress-induced breakage of the wire bond on repeated thermal cycling. High brightness LEDs can have heat loads on the order of 100 Watts per square centimeter. Since the coefficients of thermal expansion of epoxy resins typically used as encapsulants are significantly larger than those of the semiconductor layers and the moduli of the epoxies can be high, the embedded wire bond can be stressed to the point of failure on repeated heating and cooling cycles.
Thus, there is a need for new photochemically stable and thermally stable encapsulants for LEDs that reduce the stress on the wire bond over many temperature cycles. In addition, there is a need for encapsulants with relatively rapid cure mechanisms in order to accelerate manufacturing times and reduce overall LED cost.